1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and method for generating a plurality of echoes by applying a 90.degree. RF pulse for magnetically exciting spins and sequentially and repetitively applying a 180.degree. RF pulse.
2. Description of the Related Art
The spin echo imaging (SE) method generates a spin echo on the following principle. After the application of a 90.degree. RF pulse, the spin phases are diffusively varied after a lapse of time. This is called a transverse relaxation phenomenon. When a 180.degree. RF pulse is applied after a lapse of time of .tau./2, the phases of respective spins are focused after a lapse of time of .tau. from the application of the 90.degree. RF pulse and a transient peak emerges at a free induction decay (FID), thus generating spin echoes.
With the fast spin echo imaging method (hereinafter referred to as an FSE method), a plurality of spin echoes can be obtained by applying a 90.degree. RF pulse and repetitively applying a 180.degree. RF pulse. The FSE method, compared with the SE method, can reduce the application times of the 90.degree. RF pulses drastically. In comparison with the SE method, the FSE method can achieve faster imaging.
The FSE method allows no flexibility upon the designing of a pulse sequence under the following rule. That is, according to the rule above, every interval of a 180.degree. RF pulse can be uniformalized to .tau., and the echo time from the application of the 90.degree. RF pulse until a first echo is obtained, as well as every echo interval of those echoes E2, E3, . . . , emerging after the appearance of the first echo, is uniformalized to .tau.. The rule is based on the following.
It is not possible, in reality, to make a 180.degree. RF pulse have 180.degree. flip angle components only. For this reason, the magnetized spin is dissolved, upon each reception of a 180.degree. RF pulse, into a first component undergoing a phase inversion as expected, a second component undergoing a longitudinal magnetization and a third component undergoing a direct steady phase spread without being given any influence by the 180.degree. RF pulse. The first component emerges as a primary echo, the second component as a stimulated echo and the third component as an indirect echo, by the stimulated echo it is meant an echo generated, through the longitudinal magnetization, after the application of the 90.degree. RF pulse, for example, an echo which, under a varying phase spread of spins along an A-B-E-F path in FIG. 1, is generated when the spin phase focuses at a 0 point. By the indirect echo it is meant an echo generated upon being unusually subjected by a 180.degree. RF pulse to phase inversion, for example, an echo which, under a varying phase inversion along an A-B-D-G-K path, is generated when the spin phase focuses at a 0 point. These stimulated and indirect echoes, together with the primary echo, are effective to composing an image. In order to utilize the stimulated and indirect echoes for composing an image, it has been necessary to generate stimulated and indirect echoes in the same timing as the primary echo and it has, therefore, been necessary to make, constant, all RF pulse intervals and their echo intervals as set out above. Paradoxically, if the stimulated and indirect echoes are generated in a different timing from the primary echo, then this results in their mutual interference and in a poor image quality.
From this situation in which the FSE method, being restricted under the above-mentioned rules, will not allow any flexible pulse sequence, the following disadvantages are reduced. For example, the FSE method enables a plurality of echoes to be obtained in the transverse time T2 and, therefore, the time and interval of an initial echo are shortened compared to the SE method. For a short echo time, there is no decline of a fat signal involved. It is, therefore, not possible, according to the FSE method, to obtain an image of a T2 contrast substantially equal to that obtained according to the SE method. If the echo time is lengthened so as to achieve a decline in the fat signal, subsequent echo intervals are also lengthened, thus resulting in a longer data collection time or in a decline in the number of echoes involved. In order to achieve such a decline in the fat signal without lengthening the echo time and echo interval, the techniques, such as the presaturation processing or the elimination of the fat signal through a chemical shift, have been developed, but there arise the problems with an increase in an SAR (RF exposure), decline in an S/N ratio, decline in the number of echoes resulting from the addition of prepulses, inhomogeneity of a static magnetic field, and so on. These problems must be corrected with high accuracy.
Let it be given that the FSE method and its applied GRASE (gradient and spin echo) method are applied to a dual contrast mode by which it is possible to obtain two kinds of image of different contrast, a proton density weighted image and T2 weighted image, from the first half's echoes and latter half's echoes. In this case, the S/N of the latter half's echoes is lower than that of the first half's echoes in view of its longer relaxation time and the image quality is extremely unbalanced among the images. In order to improve the S/N ratio on the latter half's echoes and eliminate an unbalance among the image quality, it is necessary to lengthen the echo collection time of the latter half's echoes and, by so doing, narrow down the echo collection band. However, the above-mentioned rule does not allow only the echo collection time of the latter half's echoes to be lengthened while the echo collection time of the first half's echoes is reserved in an "as shortened" way.
Let it be given that the first half's echoes and latter half's echoes are collected by the FSE method and GRASE method, respectively. When the FSE method and GRASE methods are used separately, the 180.degree. RF pulses are applied in different time intervals. However, the FSE method and GRASE method, being used in combination, are so uniformalized as to allow the use of a 180.degree. RF pulse interval.